There is a large market for security systems. In a typical system, security-monitoring sensors are installed throughout a residential or commercial location. These sensors are of various kinds, with some of them looking for unauthorized intrusions, smoke, spills, or other mishaps. When a sensor “triggers,” that is, when the sensor notices something untoward, it usually alerts a central control station on the premises. The central control station may then raise an alarm to alert people on the premises. The central control station may also contact remote security personnel, depending upon the nature of the sensed disturbance. For example, when a smoke sensor triggers, the central control system may sound a local alarm and call the fire department (or call a security service that in turn calls the fire department).
These security systems are becoming more valuable as the types of sensors proliferate. While many of these sensors are relatively inexpensive, installation can be very expensive. Power and communications lines must be run to each wire-line sensor. When the configuration of the premises changes (more often an issue with commercial installations than with homes), the wire-line sensors must be repositioned and re-wired, and the whole system may need to be reconfigured so that the central control system understands the new layout. Wiring costs can be lowered by the use of sensors that run on batteries and that communicate wirelessly with the central control station. While the installation of these wireless sensors is cheaper than installing wire-line sensors, replacing batteries can be a burdensome chore, especially for a large commercial installation that may have hundreds of sensors.